Axford and Hines visualized a magnetosphere whose field filled a cavity in the solar wind, elongated on its night side into a tail, as previously suggested by Johnson [1960], so that all field lines emanating near the magnetic pole extended into the tail. Their proposed convective flow pattern (Figure 5a, from Hill [1983])) carried plasma tailward along the flanks and returned it by means of a sunward flow near the x-axis, skirting around the region closest to Earth. They furthermore suggested that such a flow could be caused by a viscous-like momentum transfer from the solar wind to adjacent regions of the tail, although they admitted that other processes could produce similar flows in the polar cap, including Dungey's reconnection scenario (further below).

When the flow pattern of Figure 5a is mapped along field lines to the polar ionosphere, it produces a two-cell flow pattern, with plasma streaming nightwards across the pole and returning to the day side at lower latitudes, with flow lines similar to the contours in Figure 5b. By equation (1), if E =-grad V, it follows that v/gradV = 0 and therefore the plasma flow lines in Figure 5b are also lines of constant electric potential F, suggesting a dawn-to-dusk electric field across the polar caps. Satellites in a low-altitude polar orbit can observe such a field directly, by measuring the small voltage difference between the tips of a long antenna [Aggson, 1968; Cauffman and Gurnett, 1972]. The first satellites to successfully conduct such observations were Iowa's Injun 5 [Cauffman and Gurnett, 1971] and the OGO-6 observatory [Heppner, 1972a, 1977]; some later missions, e.g. Atmosphere Explorer 1 and Dynamics Explorer 2, measured E indirectly using "driftmeters" which observed v through the anisotropy of particle fluxes caused by the plasma's bulk motion [Hanson and Heelis, 1975; Heelis et al., 1981].

The observations confirmed the two-cell pattern and obtained typical voltage drops of 40-70 kV; this agreed with a prediction of the reconnection model by Levy et al. [1964; sect. VI]. Much depends upon the state of the interplanetary magnetic field (IMF). The two-cell pattern is most stable when the IMF has a southward slant (Bz < 0, see below), and it contains asymmetries correlated with the dawn-dusk By component of the IMF [Heppner, 1972c; Heppner and Maynard, 1987]. The cross-polar voltage drop DF on the average grows with southward Bz, though individual observations fluctuate greatly. With northward Bz the average DF sometimes decreases to less than 20 kV, and it has been suggested that at such times it may "bottom out" at a low level contributed by a viscous-like interaction at the flanks [Reiff et al., 1981; Wygant et al., 1983].

When the IMF has a northward slant, the 2-cell pattern becomes distorted and E is often irregular. At times non-standard patterns may develop, such as the 4-cell pattern deduced by Burke [1979]. More complex patterns have also been claimed [Reiff and Burch, 1985] but they are hard to confirm without simultaneous passes by a fairly large number of satellites.

In the innermost magnetosphere the plasma density n is dominated by the thermal ionospheric plasma which tends to co-rotate with Earth; this is another consequence of field line sharing [Ferraro, 1937] and is enforced by a corotation electric field ECR, which near Earth is much larger than the convection field E. It was found from whistler wave observations [Carpenter, 1963; Carpenter and Park, 1973] and later by in-situ observations that n often dropped precipitously from about 20-100 ions/cc to about 5 ions/cc at a "plasmapause" boundary on field lines that extended to 4-5 RE. Brice [1967; Kennel, 1985] and Nishida [1966] proposed that this was essentially the boundary of the region where low energy plasma shared the rotation of the Earth; beyond it the convection electric field E overpowered ECR. This view is now widely accepted, although the suggestion was also made [Lemaire, 1975] that the plasmapause was the limit beyond which low-energy plasma was easily lost through the interchange instability, an approach first explored by Brice [1973].